It sounds like you are looking for overall guidance on project management of your code? GitLab offers very robust user and group management options that should be able to fit your use case.
Are you familiar with version control systems as a whole / currently using one?
I believe you can achieve what you’re looking for by using features such as User / Group roles and Protected Branches.
Do you have a Project up (even a dummy one) that we could use as a test case for what you’re trying to accomplish?
If you’re looking for a very specific outline of the workflow for GitLab and version control in general, check out this video: https://www.youtube.com/watch?v=enMumwvLAug